The present invention relates generally to optical ice detection, and in particular to a system and method of ice detection using quasi-optical radar.
An Optical Ice Detector (OID) may be configured to probe the airstream surrounding an aircraft to determine the properties of the clouds through which the aircraft is passing. Prior art OIDs utilize near-infrared beams with wavelengths that lie near 1 μm, which is less than the diameter of most water droplets and ice crystals within clouds. Because of this, the light scattering from the cloud is primarily due to Mie scattering for water droplets and geometric scattering for larger ice crystals. For Mie scattering caused by droplets larger than 3-4 μm, the scattering efficiency is nearly constant, and the scattering cross-section increases in proportion with the cross-sectional areas of the water droplets. Even though large droplets produce a backscatter signal greater than small droplets, the abundance of small droplets compared to the scarcity of large droplets in a cloud often causes the backscatter to be dominated by small droplet scattering.
For clouds in which the droplet number density with respect to diameter follows a mono-modal statistical distribution, the dominance of small-droplet backscatter creates no issue for cloud characterization. The mean or mode and the distribution shape parameter as derived from the backscatter signal allow determination of the number density of large droplets. For droplet size distributions with multiple modes, however, the presence of anomalous amounts of large droplets in a secondary mode can be difficult to detect. Such conditions may occur, for example, when cumulus clouds drizzle or rain into a lower stratiform cloud deck. If the temperature is below freezing, supercooled large droplets (SLD) that strike the leading edge of a wing can run back past icing protection systems and affect the aerodynamics of the aircraft. Thus, it is desirable for an aircraft crew to detect these SLDs during flight.